The present invention relates to a magnetic recording and reproducing apparatus (hereinbelow, termed "VTR") which records a video signal and a sound signal and reproduces them by the use of a magnetic tape.
In a prior-art VTR, in recording a video signal and a sound signal on a magnetic tape, the received video signal and sound signal are respectively subjected to signal processing, whereupon the processed signals are recorded on different places (tracks) of the magnetic tape through respective exclusive heads. FIG. 8 shows a block diagram of the prior-art magnetic recording and reproducing apparatus, while FIG. 9 shows a recorded pattern on the magnetic tape in the prior art. They will now be explained. Referring to FIG. 8, input sound signals 1 and 1' have a bias signal of several tens kHz superposed thereon in a sound signal processing circuit 2 and are recorded on the magnetic tape through sound heads 3 and 4. Here, the sound signal inputs 1 and 1' and the sound heads 3 and 4 are respectively supplied with an R signal and an L signal for stereo sounds or with signals corresponding to a sound-main signal and a sound sub-signal, whereby stereophony or sound multiplex broadcast is coped with.
On the other hand, in the mode of reproduction, signals detected by the sound heads 3 and 4 are subjected to signal processing in the sound processing circuit 2, and reproduced sound signals 5 and 5' respectively corresponding to the input sound signals 1 and 1' are obtained.
A received video signal 6 is subjected to signal processing by a video signal processing circuit 7, and the processed signal is supplied through a rotary transformer 8 to video heads 9 and 10 which are mounted on a rotating cylinder. The video heads 9 and 10 are installed symmertically to each other with respect to the axis of rotation and with a certain inclination to the traveling direction of the magnetic tape. They move on the magnetic tape while being alternately switched, to magnetically record or reproduce the video signal. Thus, a reproduced video signal 15 is obtained.
Owing to the operations stated above, as shown in FIG. 9, the respective sound signals are recorded on recording tracks 11 and 12 by the sound heads 3 and 4, and the video signal is recorded on recording tracks 13, 13', 13" . . . and recording tracks 14, 14', 14" . . . by the video heads 9 and 10 respectively.
In recent years, the magnetic recording and reproducing characteristics of video signals have been enhanced with improvements in magnetic heads and magnetic tapes. Therefore, the quantity of a tape required for attaining a desired signal-to-noise ratio has been decreased. Accordingly, the tape speed has been lowered to enhance the recording density. In this case, however, the technique of the exclusive track recording system as described above has led to the disadvantage that regarding the sound signals, the sound heads are fixed, so the bands of the reproduced sound outputs narrow in correspondence with the lowered component of the tape speed, and the signal-to-noise ratio degrades.
Further, difficulties have been involved as to wow and flutter characteristics due to the instability of a tape running system.
In order to relieve or eliminate the disadvantages of the above technique, the following techniques of a recording system which uses a video track for a double purpose have been proposed. The first method consists in utilizing a method disclosed in the Official Gazette of Japanese Patent Application Publication No. 39-9565. The method described in this official gazette is such that a video carrier subjected to frequency modulation with a video signal and a sound carrier subjected to frequency modulation with a sound signal are both recorded on a recording track for the video signal. The official gazette contains quite no description on a case where the video signal is a color signal. However, when the disclosed method is utilized, the recording of the color video signal is carried out as follows.
First, the sound carrier subjected to frequency modulation with the sound signal is recorded on the recording track for the video signal. Subsequently, a luminance signal carrier subjected to frequency modulation with a luminance signal and also a down-converted chrominance signal are superposedly recorded on the same track as the track on which the sound carrier has been recorded. The sound signal is first recorded, and the video signal is subsequently superposed thereon. The signal subjected to frequency modulation with the sound signal is recorded down to the depths of the magnetic layer of the magnetic tape, whereupon the signal subjected to frequency modulation with the luminance signal, which is higher in frequency than the signal subjected to frequency modulation with the sound signal, is recorded on the surface of the magnetic layer without being erased.
As the second method, the signal modulated with the sound signal is provided between the band of the down-converted chrominance signal and the band of the signal modulated with the luminance signal, and these three signals are multiplexed frequency-wise, whereby the sound signal is recorded on the recording track for the video signal.
Thus, the relative speed between the tape and the head is sharply raised, and the signal-to-noise ratio of the reproduced sound quality can be enhanced.
The recording system using the video track for the double purpose, however, has had the disadvantage that when two FM sound carrier frequencies are set in correspondence with the stereophonic R signal and L signal or with the sound main-signal and sub-signal, a beat attributed to both the carrier frequencies develops and appears in the band of the down-converted chrominance signal, so it leaks to the down-converted chrominance signal through the tape of the same recording track or the rotary transformer 8 to degrade the reproduced color picture quality. More specifically, as illustrated in FIG. 10, regarding the down-converted chrominance signal 15, the signal 16 frequency-modulated with the luminance signal, and the FM sound carriers 17 and 18, the beats 19 and 20 appear in the band of the down-converted chrominance signal in the reproducing mode, to hamper the reproduced color signal. Especially in the absence of the sound signal, the beats arise fixedly on a screen, and the degradation is conspicuous.
Here, the latter method of the frequency-wise multiplexing is comparatively less susceptible to the nonlinearity of the tape/head system because the signal modulated with the luminance signal has a bias effect on the signal modulated with the sound signal. In contrast, in the former method of the superposed recording on the track, there is no bias signal for the signal modulated with the sound signal, the influence of the nonlinearity is great, and the beats of particularly odd-numbered orders ascribable to the nonlinearity are great.